The present invention relates generally to antennas for use on spacecraft, and more particularly, to variable beamwidth and zoom contour beam antenna systems designed for use on spacecraft.
The present invention relates to improvements in offset Gregorian reflector antenna systems for use on communication satellites. Due to unpredictability of communication traffic, it is desirable that the beamwidth of the antenna radiation pattern be changeable when the spacecraft is on orbit.
When a satellite is in an elliptical orbit, the view of the coverage area (CONUS, for example) from the satellite changes as the satellite travels. The size of the coverage area viewed from the satellite is inversely proportional to the distance between the satellite and the earth. Under such circumstances, in order that a communication system on the satellite is to be operated efficiently, it is required that (1) the antenna radiates a contour beam (the beam contour matches the boundary of the coverage) and (2) the contour beam is variable or zoomable (size of the beam changes) in space.
Previously known zoom antennas are limited to radiate (1) a circular beams (not a contour beam) wherein the radiation contour of antenna matches the boundary of the coverage area, such as is disclosed in U.S. patent application Ser. No. 09/531,613, filed Mar. 21, 2000, assigned to the assignee of the present invention, and (2) a defocused elliptical beam, not a zoomed contour beams such as is disclosed in U.S. Pat. No. 5,977,923, entitled xe2x80x9cReconfigurable, zoomable, turnable, elliptical-beam antennaxe2x80x9d, by Contu, et al, issued Nov. 2, 1999.
Accordingly, it would be advantageous to have improved variable beamwidth and zoom contour beam antenna systems that may be used on a spacecraft that has a changeable beamwidth. It would also be advantageous to have improved zoom contour beam antenna systems that radiate a contour beam, and wherein the contour beam is variable or zoomable.
The variable beamwidth antenna system comprises a main reflector, a reflector displacement mechanism, a subreflector, a feed horn, and a feed horn displacement mechanism. The reflector displacement mechanism can place the main reflector at any desired location, while the feed displacement mechanism can place the feed horn at any desired location. More specifically, the main reflector displacement mechanism controls the spacing between the main reflector and the subreflector; while the feed displacement mechanism controls the spacing between the feed and the subreflector.
To implement the present invention, two mechanical movements are required. First, the RF feed horn and the subreflector are moved closer together. Second, the main reflector is moved away from the subreflector. Alternatively, the subreflector is moved closer to the RF feed horn, and the main reflector is moved away from the subreflector.
Two mechanical movements are not independent. They are related by   y  =                    d        2            ⁢      x                      c        2            -              x        ⁡                  (                      c            +            d                    )                    
where xe2x80x9cxxe2x80x9d is the distance of RF feed horn displacement, and xe2x80x9cyxe2x80x9d is the distance of the main reflector displacement.
When x=y=0, the antenna arrangement is in the focused condition. Under the focused condition, the focal point of the paraboloidal main reflector is coincident with one of the foci of the elliptical subreflector, and the feed horn is located at the other focus of the subreflector. The terms xe2x80x9ccxe2x80x9d and xe2x80x9cdxe2x80x9d in the above equation are the distance between the RF feed horn and the subreflector, and the distance between the focal point of the main reflector and subreflector, respectively, when the antenna is in focus.
A reduced-to-practice embodiment of the present variable beamwidth antenna has 3 dB beamwidth that can be changed while the spacecraft is on orbit by proper displacements of any two components among the main reflector, the subreflector and the feed.
Another embodiment of the present invention comprises a zoom contour beam antenna system that radiates a contour beam and whose beam is variable or zoomable. The present invention produces a zoom contour beam, wherein the radiation contour of antenna matches the boundary of the coverage area. This is possible because the present invention utilizes two mechanical displacement mechanisms respectively coupled to a main shaped reflector and a shaped subreflector that are moveable to provide zoomable contoured beams. The zoom contour beam provides higher gain in the coverage area, and thus improves the efficiency of the satellite communication system in which it is employed.
More particularly, an exemplary zoom contour beam antenna system comprises a shaped subreflector, a shaped main reflector. a feed horn, a main reflector displacement mechanism coupled to the shaped main reflector, and a subreflector displacement mechanism coupled to the shaped subreflector. The main reflector displacement mechanism repositions the main reflector, and the subreflector displacement mechanism repositions the subreflector. Thus, the subreflector displacement mechanism adjusts the position of the subreflector relative to the feed horn, and the main reflector displacement mechanism adjusts the position of the main reflector relative to the subreflector in order to zoom the beam produced by the antenna system. the repositioning of the various components is done in accordance with a predetermined displacement equation.
Thus, in accordance with the teachings of the present invention, a Gregorian reflector antenna may be used in circumstances where the desired antenna radiation pattern(s) is (are) required to be broadened (changed). Both the main reflector and the subreflector are attached to mechanical devices (displacement mechanisms) such that the reflectors can be displaced. The displacements of the two reflectors are not independent, and are related according to the displacement equation. The present invention is not limited to systems having a single pencil beam antenna, and may be used in circumstances involving multiple beam (multiple feed horn) antennas. The present invention may also be employed with a contoured beam antenna. In such a contoured beam antenna application, both the main reflector and subreflector are shaped to obtain a desired (for example, CONUS) radiation pattern.